Comment 5 for bug 1831280

I note you're using 16.04 which was upgraded from a previous install. Sometimes over time, packages can go missing, typically accidentally removed. One way you can make sure your system is up to date with the latest packages, and any missing ones are installed, is with the following commands. I'd recommend running them in a terminal, one after the other then restart the system and see if the issue still manifests itself.

sudo apt update
sudo apt install ubuntu-desktop^
sudo apt dist-upgrade

Note the second line has a ^ on the end, this is intentional, and may help to re-instate any missing packages. Hope that helps. Let us know.